Move settable documentation from source code to api docs

Author: kotfu

cmd2/cmd2.py

@@ -222,12 +222,6 @@ def __init__(self, completekey: str = 'tab', stdin=None, stdout=None, *,
                 'quiet': "Don't print nonessential feedback",
                 'timing': 'Report execution times'
             }
-        """This dictionary contains the name and description of all settings available to users.
-
-        Users use the :ref:`features/builtin_commands:set` command to view and
-        modify settings. Settings are stored in instance attributes with the
-        same name as the setting.
-        """
 
         # Commands to exclude from the help menu and tab completion
         self.hidden_commands = ['eof', '_relative_load', '_relative_run_script']

docs/api/cmd.rst

@@ -12,3 +12,11 @@ cmd2.Cmd
     .. attribute:: default_error
 
       The error message displayed when a non-existent command is run.
+
+    .. attribute:: settable
+
+        This dictionary contains the name and description of all settings available to users.
+
+        Users use the :ref:`features/builtin_commands:set` command to view and
+        modify settings. Settings are stored in instance attributes with the
+        same name as the setting.